1. Field of the Invention
The present invention relates generally to sensing a detectable environmental condition, and, in particular, to sensing moisture using an RFID system.
2. Description of the Related Art
In general, in an RF communication system, a single antenna structure is adapted to receive signals, the carrier frequencies (“fC”) of these signals can vary significantly from the resonant frequency (“fR”) of the antenna. The mismatch between fC and fR results in loss of transmitted power. In some applications, this may not be of particular concern, but, in others, such as in RF identification (“RFID”) applications, such losses are of critical concern. For example, in a passive RFID tag, a significant portion of received power is used to develop all of the operating power required by the RFID tag's electrical circuits. In such an application, a variable impedance circuit can be employed to shift the fR of the RFID tag's receiver so as to better match the fC of the transmitter of the system's RFID reader. A single design that is useful in all systems is precluded by the lack of standards as to appropriate RFID system frequencies, and, the breadth of the available frequency spectrum is quite broad: Low Frequency (“LF”), including 125-134.2 kHz and 140-148.f kHz; High-Frequency (“HF”) at 13.56 MHz; and Ultra-High-Frequency (“UHF”) at 868-928 MHz. Compounding this problem is the fact that system manufacturers cannot agree on which specific fC is the best for specific uses, and, indeed, to prevent cross-talk, it is desirable to allow each system to distinguish itself from nearby systems by selecting different fC within a defined range.
Attempts have been made to improve the ability of the RFID tag's antenna to compensate for system variables, such as the materials used to manufacture the RFID tag. However, such structural improvements, while valuable, do not solve the basic need for a variable impedance circuit having a relatively broad tuning range.
Shown in FIG. 1 is an ideal variable impedance circuit 100. Circuit 100 comprised of a variable inductor 102, a variable capacitor 104 and a variable resistor. When used as a tank in a resonant system, the circuit 100 exhibits a quality factor (“Q”) of:
                    Q        =                                            f              R                                      Δ              ⁢                                                          ⁢              f                                =                                    1              R                        ⁢                                          L                C                                                                        [        1        ]                            where: Q=the quality factor of circuit 100;        fR=the resonant frequency of circuit 100, measured in hertz;        Δf=the bandwidth of circuit 100, measured in hertz at −3 db        R=the resistance of resistor, measured in ohms;        L=the inductance of variable inductor 102, measured in henries; and        C=the capacitance of capacitor, measured in farads.        
In such a system, the resonant frequency, fR, of circuit 100 is:
                              f          R                =                  1                      2            ⁢                                                  ⁢            π            ⁢                          LC                                                          [        2        ]            
As is well known, the total impedance of circuit 100 is:
                    Z        =                                            Z              L                        ⁢                          Z              C                                                          Z              L                        +                          Z              C                                                          [        3        ]                            where: Z=the total impedance of circuit 100, measured in ohms;        ZL=the impedance of variable inductor 102, measured in ohms; and        ZC=the impedance of capacitor, measured in ohms.        
As is known, the relationship between impedance, resistance and reactance is:Z=R+jX  [4]                where: Z=impedance, measured in ohms;        R=resistance, measured in ohms;        j=the imaginary unit √{square root over (−1)}; and        X=reactance, measured in ohms.        
In general, it is sufficient to consider just the magnitude of the impedance:|Z|=√{square root over (R2+X2)}  [5]
For a purely inductive or capacitive element, the magnitude of the impedance simplifies to just the respective reactance's. Thus, for variable inductor 102, the reactance can be expressed as:XL=2πfL  [6]
Similarly, for capacitor, the reactance can be expressed as:
                              X          C                =                              1            ⁢                                                                      2            ⁢                                                  ⁢            π            ⁢                                                  ⁢            fC                                              [        7        ]            
Because the reactance of variable inductor 102 is in phase while the reactance of capacitor is in quadrature, the reactance of variable inductor 102 is positive while the reactance of capacitor is negative. Accordingly, a desired total impedance can be maintained if a change in inductive reactance is offset by an appropriate change in capacitive reactance.
Within known limits, changes can be made in the relative values of variable inductor 102, capacitor, and resistor to adjust the resonant frequency, fR, of circuit 100 to better match the carrier frequency, fC, of a received signal, while, at the same, maximizing Q.
In many applications, such as RFID tags, it may be economically desirable to substitute for variable inductor 102 a fixed inductor 202, as in the variable tank circuit 200 shown in FIG. 2. In general, in order to maximize Q in circuit 200.
The amplitude modulated (“AM”) signal broadcast by the reader in an RFID system will be electromagnetically coupled to a conventional antenna, and a portion of the current induced in a tank circuit is extracted by a regulator to provide operating power for all other circuits. Once sufficient stable power is available, the regulator will produce, e.g., a power-on-reset signal to initiate system operation.
Tags based on conventional chips can be detuned by a variety of external factors, most commonly by proximity to liquids or metals. Such factors can change the impedance characteristics of a tag's antenna. When the tag chip has a fixed impedance, a mismatch between the chip and the antenna results, reducing the tag's performance.